Door Prize

Been a carpenter-mason all my grownup years, but something's been bugging this brain between my ears. I've asked experts in architecture and furniture, but the more they explain, all the more I'm unsure. I must have been to them either a pest or a bore, but then my question simply is: "What is a door?" Some say it's that panel or board that's hinged to the wall that you push or pull, swing or knock on when you call. With knob and key, some say the door secures the place, but to others, it's nothing but that vertical space through which you can walk or dash away from pole to pole; as a passage through a wall, a door is just a hole. But without that panel, you'll be knocking on nothing and if you do, folks will think you're nuts or something; but then without that gap or hole we call empty space, that wall will flatten out that nose upon your face. Today, I don't ask that silly question anymore for when I do, I'm shooed away and shown the door!